As such an ink follower composition for ballpoint pen (also referred to as "liquid stopper" or "a back flow preventing composition"), a mixture of a mineral oil and a metallic soap (i.e., grease) has been used in ballpoint pens using high-viscosity oily ink.
When applied to ballpoint pens using middle-viscosity oily ink or shear-thinning aqueous ink, the ink follower applicable to the high-viscosity oily ink does not follow the ink satisfactorily or fails to prevent the ink from flowing backward because of its large viscosity change with temperature. Hence, various gelatinous polybutene-based ink followers comprising a hardly volatile organic liquid, typically exemplified by liquid polybutene and a lipophilic gelatinizer added therein have been proposed and put to practical use broadly in ballpoint pens using the aforementioned ink.
Conventional lipophilic gelatinizers include dibenzylidene sorbitol, tribenzylidene sorbitol (see an examined published Japanese patent application (kokoku) 1-10554, N-acylamino acid derivatives (see an unexamined published Japanese patent application (kokai) 57-200472, and clay which is surface-treated with an onium compound having a long-chain alkyl group (see an examined published Japanese patent application (kokoku) 7-17872 corresponding to U.S. Pat. No. 4,671,691).
However, if the polybutene-based ink follower has a low molecular weight or if the amount of the gelatinizer is small, there is a tendency that a part of the ink follower is separated (liquid separation) with time. While ballpoint pens having such a polybutene-based ink follower are allowed to stand for a long period of time with their tips upward as in the case where they are displayed for sale, the separated part of the ink follower, which has a smaller specific gravity than the ink, tends to rise along the inner wall of the ink tube and jam up at the tip of the ballpoint pen, which would result in poor start of writing.
On the other hand, if the polybutene in the follower has a high molecular weight, or if the gelatinizer is added in an increased amount, the ink follower that should follow the ink with the ink consumption tends to be too hard to follow or be adhered to the inner wall of the ink tube and decrease in amount as it moves with the ink consumption, resulting in adverse influences on various performance properties. Further, these unfavorable phenomena are easily influenced by temperature change. In other words, preparation of the polybutene-based ink follower, i.e., the molecular weight of polybutene and the amount of the gelatinizer to be added, should be delicately controlled, and the proper application of the polybutene-based ink follower has been difficult.
Another problem of the polybutene-based ink follower is that its viscous structure tends to be destroyed by the force imposed on charging in an ink tube in the production of ballpoint pens. It is hard to restore the once destroyed viscous structure, or the restoration requires a very long time, or the restored ink follower tends to have an increased viscosity over the initial one. These phenomena cause considerable reduction in writing performance of ballpoint pens.
The present invention is to eliminate the above-mentioned disadvantages of conventional ink followers for ballpoint pens. That is, an object of the present invention is to provide an ink follower composition for a ballpoint pen which has satisfactory properties of following a middle- to high-viscosity ink or shear-thinning ink, is hardly influenced by temperature change, and can be produced at good productivity and also to provide a ballpoint pen using such ink follower.